1. Field of the Invention
The present invention relates to surface light source devices of side-light type used for liquid crystal displays and the like and, more particularly, to a surface light source device of side-light type with emitting directivity utilizing a light scattering guide plate which decreases in thickness as distance increases from a primary light source.
2. Related Art
Surface light source devices referred to as "side-light type surface light source devices" have conventionally been known and are used, for example, for back-lighting of liquid crystal displays. A surface light source device of side-light type has a light guide plate and a primary light source disposed on one side thereof. A rod-shaped primary light source element such as a cold-cathode ray tube is used as the primary light source. Illuminating light emitted from the primary light source enters the light guide plate via an incidence end surface of the light guide plate, deflected therein, and emitted from a flat surface of the light guide plate to be used for illuminating a liquid crystal panel or the like.
Since the primary light source of a surface light source device of side-light type is arranged on one side of the light guide plate, the arrangement for back-lighting of liquid crystal panel can avoid increasing in thickness.
As a material for a light guide plate incorporated in a surface light source device of side-light type, a light scattering guide element having scattering power therein is more advantageously used than a transparent material from the viewpoint of emission efficiency of illuminating light. Such a light guide plate is referred to as "light scattering guide plate". Sheet-like shapes with a substantially uniform thickness and shapes gradually decreasing in thickness with the distance from a primary light source may be employable. In general, the latter is more advantageous than the former from the viewpoint of emission efficiency of illuminating light.
FIG. 1 is an exploded perspective view showing a general configuration of a side-light type surface light source device of the latter type utilizing a light scattering guide plate. A side-light type surface light source device 1 has a light scattering guide plate 2 and a primary light source 3 disposed on one side thereof. The light scattering guide plate 2, a reflection sheet 4 and a prism sheet 5 are disposed in a laminated state. The primary light source 3 comprises a cold-cathode tube (fluorescent lamp) 6 and a reflector 7 which partially surrounds the tube and consists of a reflection member having a substantially semi-circular sectional configuration. Illuminating light emitted from the open side of the reflector 7 is incident upon an incidence end surface of the light scattering guide plate 2.
The reflection sheet 4 consists of a sheet-shaped regular reflection member made of metal foil or the like, alternatively a sheet-shaped irregular reflection member made of a white PET film or the like.
The light scattering guide plate 2 consists of a light scattering guide element having a wedge-like sectional configuration. A light scattering guide element is an optical material having a light-guiding function and a volumetric scattering function which is formed, for example, by uniformly dispersing light transmittable fine particles in a matrix made of poly methyl methacrylate (PMMA), the particles having a refractive index different from that of the matrix.
In the surface light source device of side-light type shown in FIG. 1, as indicated in FIGS. 2 and 2A by a section taken along the line I--I shown in FIG. 1, illuminating light L is introduced into the light scattering guide plate 2 through an incidence end surface T which is an end surface facing the primary light source 3. The illuminating light L propagates through the light scattering guide plate 2 while being repeatedly reflected between a flat surface facing the reflection sheet 4 and a flat surface facing the prism sheet 5 (hereinafter referred to as "emission surface") under a scattering effect provided by the light transmittable fine particles. Such a reflection sheet 4 employed, the illuminating light L is further subject to an irregular reflection effect.
As the angle at which the illuminating light L is incident upon the emission surface gradually decreases during the propagation, and components with incident angles equal to or smaller than the critical angle relative to the emission surface are emitted from the emission surface. Illuminating light L1 emitted from the emission surface is emitted as scattered light as a result of scattering caused by the light transmittable fine particles in the light scattering guide plate 2 and irregular reflection caused by the reflection sheet 4. However, the principal emission direction of the illuminating light L1 is inclined toward the top end of the wedge-like configuration relative to the front direction, as shown in an enlarged illustration indicated by the arrow B.
In other words, the emitted light L1 has directivity, thereby providing the side-light type surface light source device 1 with emitting directivity.
The prism sheet 5 is formed of a light transmittable sheet material such as polycarbonate and is formed with a prism surface on the side thereof facing the light scattering guide plate 2. This prism surface includes protrusions, repeatedly formed from the incidence end surface T toward the end of the wedge-like configuration, having a triangular sectional configuration extending substantially in parallel with the incidence end surface T of the light scattering guide plate 2. The prism sheet 5 corrects the principal emission direction of the emitted light L1 so as to direct it substantially forwardly of the emission surface at inclined surfaces of the triangular protrusions.
As a result, this side-light type surface light source device 1 provides a characteristic, such that a more efficient frontal emission is realized compared with a side-light type surface light source device of a type in which a light scattering guide plate is formed with a substantially uniform thickness.
Side-light type surface light source devices with emitting directivity include those comprising a light transmittable member or translucent member to form a light scattering guide plate with a wedge-like configuration or a configuration similar to a wedge-like configuration and having a scattering film or the like formed on the emitting surface or rear surface thereof.
In such a side-light type surface light source device with emitting directivity, linear portions having a higher luminance level (bright lines) as indicated by the reference symbol K in FIG. 1 and band-like portions having a lower luminance level (dark bands) are produced in the form of bands at roughly constant intervals in parallel with the incidence end surface, on the emission surface near the incidence end surface. The bright lines originate from illuminating light which has entered the light scattering guide plate via the upper and lower edges of the incidence end surface. The dark bands appear between the bright lines with a width roughly corresponding to the thickness of the incidence end surface and have a luminance level higher than the average luminance of the emission surface. Such variation in luminance is sometimes referred to as "reflective appearance" and prevents a side-light type surface light source device with emitting directivity from generating high quality illuminating light.
In addition to the above-described first type of variation in luminance (reflective appearance), side-light type surface light source devices are subject to a second type of variation in luminance. Specifically, when the light scattering guide plate 2 is designed with a great width H (the longitudinal length of the fluorescent lamp 6), regions of low luminance appear around peripheral portions of the emission surface in the vicinity of the incidence end surface (see the reference symbol X). This is because electrodes 6a and 6b are formed on both ends of the fluorescent lamp 6 and, in the vicinity of those electrodes 6a and 6b, the tube has internal regions having no fluorescent material applied thereon which significantly reduce the amount of the illuminating light L emitted (emission density) in the vicinity of both ends of the fluorescent lamp 6.
Variation in luminance caused by such effects can be suppressed by making the width H of the light scattering guide plate 2 smaller than the length of the fluorescent lamp 6 to some extent (for example, by the length of the areas of the electrodes). However, the reduction in the width H of the light scattering guide plate 2 means a reduction in an effective light-emitting area, which is quite disadvantageous.
In other words, if variation in luminance caused by such effects can be effectively avoided, it will be possible to design the light scattering guide plate 2 with a width H substantially equal to the length of the fluorescent lamp 6 to provide a surface light source device which has a relatively large emission surface and which can therefore be applied to a liquid crystal display having a relatively large display panel.
That is, if a relatively short fluorescent lamp can be used for a light scattering guide plate having an emission surface of a prescribed size, the surface light source device as a whole or a liquid crystal display incorporating the same can be made compact.
As one method of overcoming the shortage of luminance around peripheral portions, for example, a method as disclosed in Japanese unexamined patent publication No. H5-88168 is known wherein a rough surface is formed in a central portion of an incidence end face, the portion being a region subjected to a maximum amount of illuminating light. Although this can reduce variation in luminance in a side-light type surface light source device formed with a light guide plate having a substantially uniform thickness, it increases variation in luminance when applied to a side-light type surface light source device with emitting directivity.
Further, U.S. Pat. No. 5,178,447 discloses configuration of an incidence end surface using a rough surface. However, the roughening of an incidence end surface is applied to a sheet-shaped transparent light guide plate having a constant thickness; the width of the light scattering guide plate is shorter than the length of the rod-shaped primary light source element used as a primary light source; and it does not disclose the idea of roughening the incidence end surface as a means for avoiding the reduction in the width of the light scattering guide plate.
Further, unexamined Japanese utility model publication No. H6-68003 discloses configuration of an incidence end surface of a wedged-shaped transparent light guide plate using a rough surface, but it does not disclose the idea of roughening the incidence end surface in an attempt to avoid the reduction in the width of a light scattering guide plate.